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REVIEW 4 major objections 5 minor 35 references

Looking for new strategies to probe low mass axion-like particles in ultraperipheral heavy-ion collisions at the LHC

T0 review · 4 major / 5 minor · reviewed 2026-08-10 · deepseek-v4-flash

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Pith's one-line read A displaced-vertex cut on photon conversions removes the light-meson background for low-mass ALP searches in ultraperipheral PbPb collisions, but current LHC heavy-ion luminosities yield only about 10^-3 expected events.

desk verdict A modest, honest feasibility study whose main negative conclusion is robust, but whose fiducial cross-sections are optimistic due to an incomplete decay-length window and unspecified per-photon conversion requirement. read the letter →

arxiv 2501.14041 v1 pith:RBWDK37S submitted 2025-01-23 hep-ph hep-ex

Pedro Nogarolli , Victor P. Gonçalves , Murilo S. Rangel This is my paper · ORCID
classification hep-phhep-ex
keywords axion-likeparticlesultraperipheralheavy-ioncollisionsdisplacedvertexsearchphotonconversiondiphotondecaysALICEdetectorLHCbequivalentapproximation
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper asks whether low-mass axion-like particles (ALPs) decaying to two photons can be found in ultraperipheral lead-lead collisions at the LHC, a region where light-meson decays normally swamp the signal. It proposes requiring the ALP decay vertex to lie outside the luminous region where the primary collision occurs, and reconstructing that displaced vertex using photons that convert into electron-positron pairs inside the detector. Using the SuperChic event generator with ALICE and LHCb detector characteristics, the authors find the light-meson background is strongly suppressed and that LHCb would probe a larger range of ALP masses and couplings. With the PbPb luminosities collected in 2018, however, the expected number of ALP events is only about $10^{-3}$ at either experiment, so the strategy is not usable with present data and would become viable only after roughly a thousand times more heavy-ion data are available.

What carries the argument

The mechanism carrying the argument is the displaced-vertex selection applied to the process $PbPb \to Pb \otimes \gamma\gamma \otimes Pb \to Pb \otimes a \otimes Pb \to Pb \otimes \gamma\gamma \otimes Pb$. The ALP decay length, $l_{\rm decay} \approx 0.1\,\text{cm}\,(p_a/0.4\,\text{GeV})(0.2\,\text{GeV}/m_a)^4(0.1\,\text{TeV}^{-1}/g_{a\gamma\gamma})^2$, is required to exceed the detector-specific luminous region, so only long-lived low-mass ALPs are kept; the photons from the decay are then required to convert into electron-positron pairs in the tracker, with 25% probability at LHCb and 8.5% at ALICE, which is what makes the secondary vertex reconstructable. The cross-section is estimated in the equivalent-photon approximation using the SuperChic v4.03 event generator, and a fiducial cross-section $\sigma_f = \sigma\, N_{\rm sel}/N_{\rm gen}$ is defined after acceptance, conversion, and displaced-vertex requirements are applied.

What would settle it

A Monte Carlo or test-beam study of photon conversions in the ALICE and LHCb trackers that computes the minimum decay length at which a two-photon vertex can be resolved, for ALP masses of 0.2 to 1 GeV, would settle the central claim: if that resolvable minimum is larger than the 0.013 cm or 0.5 cm luminous-region cut for most of the mass range, the reported fiducial cross-sections and the $10^{-3}$ event counts are overestimates.

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Extended reading notes

Core claim

The paper argues that selecting ALP decays with decay length $l_{\rm decay}$ larger than the PbPb luminous region—0.013 cm at LHCb and 0.5 cm at ALICE—eliminates the dominant background from light mesons, which decay inside the primary vertex region, and that photon conversion into electron-positron pairs provides the handle to reconstruct the displaced vertex. Under this selection, the fiducial cross-sections are largest for small ALP masses and peak at couplings $g_{a\gamma\gamma}$ around $10^{-5}\,\text{GeV}^{-1}$ at ALICE and $10^{-4}\,\text{GeV}^{-1}$ at LHCb, with LHCb's smaller luminous region and higher conversion probability giving cross-sections about an order of magnitude larger. Concretely, for the 2018 integrated luminosities of $1\,\text{nb}^{-1}$ (ALICE) and $0.25\,\text{nb}^{-1}$ (LHCb), the expected signal is on the order of $10^{-3}$ events, making the search statistically unfeasible now; the authors conclude that roughly $10^3$ times more data would be required.

Load-bearing premise

The strategy assumes that every ALP decaying beyond the luminous region can actually be reconstructed as a displaced vertex from converted photon pairs, with no demonstrated upper limit on decay length and no proof that the highly collinear pairs from boosted low-mass ALPs are resolvable.

Editorial extensions

If this is right

  • The displaced-vertex requirement removes the light-meson diphoton background, leaving a background-free signal region for low-mass ALPs in ultraperipheral PbPb collisions.
  • LHCb is the more promising detector for this search, with fiducial cross-sections about an order of magnitude larger than ALICE's because of its smaller luminous region and higher photon-conversion probability.
  • With the 2018 PbPb integrated luminosities, the expected number of ALP events decaying to two photons is on the order of $10^{-3}$ at both ALICE and LHCb, so the strategy cannot constrain ALP parameters with present data.
  • If future heavy-ion runs deliver roughly a thousand times more data, the same selection becomes a viable ALP search channel.
  • The paper provides the first fiducial cross-sections for a secondary-vertex-based ALP search in ultraperipheral collisions, which can be used to plan future heavy-ion runs.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Beyond the paper: the reported event counts assume every decay that passes the luminous-region cut yields a reconstructable vertex; a detector-level simulation of material distribution and vertex resolution could show that the most collinear photon pairs from boosted low-mass ALPs are not resolvable.
  • Beyond the paper: the same photon-conversion plus displaced-vertex idea could be carried over to proton-nucleus ultraperipheral collisions, whose smaller luminous region might extend the reach to shorter ALP lifetimes for the same mass and coupling.
  • Beyond the paper: calibrating the conversion-vertex reconstruction with a known ultraperipheral diphoton process would give a data-driven check of the claimed background suppression before committing future runs to this search.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 5 minor

Summary. The paper proposes a search strategy for low-mass axion-like particles (ALPs) decaying into two photons in ultraperipheral PbPb collisions at the LHC, using ALICE and LHCb. The signal is selected by requiring the ALP decay vertex to lie outside the PbPb luminous region, with the photons identified through conversions into electron-positron pairs in the detector material. Using the SuperChic v4.03 generator, the authors compute pseudorapidity and transverse-momentum distributions and define fiducial cross-sections after acceptance and conversion requirements. They find that, with 2018 PbPb luminosities (1 nb^-1 for ALICE, 0.25 nb^-1 for LHCb), the expected number of ALP events decaying into two photons is of order 10^-3, so the strategy is unfeasible with current data; they argue that roughly 10^3 times more data would be needed and that the strategy is a valid search channel for the future.

Significance. If the quantitative predictions were reliable, the paper would provide a useful new handle for low-mass ALP searches in heavy-ion collisions, complementing existing diphoton analyses by exploiting displaced vertices and photon conversion. The paper is honest about the limited immediate sensitivity and does not overclaim. Its strengths include the use of a standard event generator, the conventional width formula, and direct use of published luminosities, with no fitting or data-tuning. The main caveat is that the detector-level treatment is schematic: the absence of an upper decay-length cut, the unspecified conversion multiplicity, and the lack of a background estimate make the quoted rates optimistic, although not in a way that changes the qualitative unfeasibility conclusion.

major comments (4)
  1. [Section III, Eq. (5)] The selection defining sigma_f in Eq. (5) imposes only the lower bound l_decay > 0.013 cm (LHCb) or l_decay > 0.5 cm (ALICE), with no upper bound ensuring that the ALP decays within the instrumented volume where photons can convert. For ma = 0.2 GeV and gaγγ = 1e-6 GeV^-1, Eq. (4) gives l_decay = 10 m, and for smaller couplings the decay length grows as gaγγ^-2; such ALPs decay far outside the tracking detector, so the resulting photons cannot produce the converted-photon secondary vertex on which the strategy relies. An upper cut L_lum < l_decay < L_det should be imposed, with experiment-specific L_det, and the low-coupling region of Fig. 5 should be recomputed. This correction lowers the rates, so it does not invalidate the qualitative 'unfeasible now' conclusion, but the quoted fiducial cross-sections are not accurate as they stand.
  2. [Section III, Fig. 5] The paper quotes photon conversion probabilities of 25% (LHCb) and 8.5% (ALICE) but does not state whether one or both photons from the ALP decay must convert in order to reconstruct the displaced vertex. Reconstructing a two-photon decay vertex requires both photon directions; if both photons must convert, the relevant efficiency is proportional to P_conv^2, not P_conv, unless the analysis explicitly demonstrates that an unconverted-photon calorimeter direction provides sufficient vertex resolution. The text should specify the conversion multiplicity and, if the linear factor was used, correct Fig. 5 and the event-number estimates. In either case the rates are lower than quoted, which only reinforces the main conclusion.
  3. [Section III, text after Fig. 4] The implementation of the decay-length sampling is not described precisely enough. The text states that l_decay is calculated using Eq. (4), but Eq. (4) is evaluated at a representative value pa = 0.4 GeV. In a generated sample the ALP momentum varies event by event, and the correct sampled quantity is l = (p_a/m_a) * (64 pi)/(g^2 m^3) for each event. If Eq. (4) was applied with a fixed pa, the acceptance as a function of pT in Figs. 3 and 4 and the normalization in Eq. (5) are distorted. The authors should specify exactly how SuperChic was modified and recompute the results with the event-by-event decay length.
  4. [Section III, event-rate discussion] The central motivation for the strategy is background suppression, but no surviving background is estimated. The argument that light mesons decay inside the luminous region removes the direct pi0/eta/eta' peaks, yet it does not address combinatorial backgrounds from two independent converted photons, nor detector-related effects such as misreconstructed conversion vertices. A quantitative background estimate, or at least a clear detector-level argument for an irreducible background, is needed before claiming that roughly 10^3 times more data would make the strategy sensitive. This omission does not affect the 'unfeasible now' conclusion, but it is load-bearing for the future feasibility claim.
minor comments (5)
  1. [Section III] The phrase 'generated with an exponential distribution exp(-ldecay)' should read exp(-l/l_decay), where l_decay is the mean decay length; as written the exponential has no scale.
  2. [Equation (4)] Equation (4) mixes units: gaγγ is expressed in TeV^-1 while the rest of the expression uses GeV. Please use a single set of units throughout to avoid the apparent factor-of-1000 ambiguities.
  3. [Figures 3-5] The axis labels in the figures, especially the vertical coupling axis in Fig. 5, are difficult to read in the provided version. Please ensure all axes and color scales are clearly labeled.
  4. [Section IV] The sentence 'which is covered in previous studies' is unclear; please rephrase to state exactly which parameter region was covered before.
  5. [General] No systematic uncertainties are assigned to the fiducial cross-sections; at minimum, the dependence on the nuclear form factor and the absorptive gap-survival factor in SuperChic should be commented on.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the cross-sections come from an independent event generator, the decay-length condition is a physical selection cut, and the final event rates are simple products of fiducial cross-section and cited integrated luminosities.

full rationale

The paper's derivation chain is self-contained and does not reduce to its inputs. The ALP production cross-section is obtained from the SuperChic v4.03 event generator (Ref. [30]) using the equivalent-photon approximation, the decay length follows from the standard two-photon width formula in Eq. (3), and the numerical example in Eq. (4) is just the relativistic boost of 1/Gamma. The search strategy applies a physical cut, ldecay greater than the quoted PbPb luminous region (0.013 cm for LHCb, 0.5 cm for ALICE), and the fiducial cross-section in Eq. (5) is a conventional definition sigma_f = sigma Nsel/Ngen, not a fitted quantity renamed as a prediction. The expected event counts are then obtained by multiplying these fiducial cross-sections by the cited 2018 ALICE and LHCb luminosities, so the central conclusion that roughly 10^-3 events are expected is a direct consequence of the generator-level cross-sections and the detector acceptance, with no parameter tuned to the predicted quantity. The only cited strategy, Ref. [23], is external prior work that the paper explicitly extends to UPCs, and the self-citations in Refs. [15,16,19] are used only to motivate the light-meson background discussion, not to support the numerical result. Possible experimental shortcomings, such as not imposing an upper bound on the decay length or not specifying whether one or both photons must convert, would reduce the quoted rates and therefore strengthen, rather than enable, the paper's own 'unfeasible at present' conclusion; these are correctness risks, not evidence of circularity.

Assumptions & free parameters 2 free parameters · 7 assumptions · 0 invented entities

All calculations rest on the EPA, the standard ALP Lagrangian, and detector performance numbers from cited references. The only hand-scanned quantities are the ALP mass and coupling, which are the physics parameters under study, not fitted to data. The paper introduces no new particles or forces.

free parameters (2)
  • ALP mass ma
    Scanned model parameter from 0.2 to 1 GeV; it sets the ALP production kinematics and the decay width. Not fitted to data.
  • ALP-photon coupling gaγγ
    Scanned model parameter; it controls the ALP width and hence the decay-length selection efficiency. Not fitted to data.
assumptions (7)
  • domain assumption Equivalent Photon Approximation factorization of the UPC cross-section, Eq. (2)
    The total cross-section is written as a convolution of photon fluxes and the gamma-gamma subprocess cross-section, a standard approximation for ultraperipheral collisions but not exact.
  • domain assumption Absorptive factor S_abs^2(b) as modeled by SuperChic v4.03
    The probability of no additional ion-ion rescattering is taken from the generator's implementation, which the paper does not independently validate.
  • domain assumption ALP couples to photons only through the interaction in Eq. (1)
    The search assumes the simplified ALP Lagrangian with only the gaγγ coupling; other couplings are neglected.
  • standard math Tree-level decay width Gamma(a->γγ) = gaγγ^2 ma^3/(64π), Eq. (3)
    Standard QFT result for the two-photon decay of a pseudoscalar, used to set the decay length.
  • standard math Exponential distribution of decay lengths
    Unstable particle decay lengths follow exp(-t/τ); the paper generates ldecay with this distribution.
  • domain assumption Displaced-vertex requirement removes the light-meson background
    The paper assumes prompt meson decays occur inside the luminous region and therefore are eliminated by the vertex cut; no background simulation is provided.
  • domain assumption Detector parameters (luminous region sizes, conversion probabilities) from cited LHCb/ALICE papers
    The analysis uses 0.013 cm and 0.5 cm luminous region sizes and 25%/8.5% conversion probabilities as external inputs.

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Cite this review

Pith. "Pith review of Looking for new strategies to probe low mass axion-like particles in ultraperipheral heavy-ion collisions at the LHC." pith.science (2026). https://pith.science/paper/RBWDK37S

@misc{pith2026250114041,
  author       = {Pith},
  title        = {Pith review of: Looking for new strategies to probe low mass axion-like particles in ultraperipheral heavy-ion collisions at the LHC},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/RBWDK37S}},
  note         = {Machine review of arXiv:2501.14041}
}
abstract

The possibility of searching for long-lived axion-like particles (ALPs) decaying into photons is investigated in ultraperipheral $PbPb$ collisions at the Large Hadron Collider (LHC). We propose a search strategy for low mass ALPs using the LHCb and ALICE experiments. The ALP identification is performed by requiring the decay vertex be reconstructed outside the region where a primary vertex is expected, which strongly suppress the contribution associated with the decay of light mesons. We also use the fact that a fraction of the photons convert into electron-positron pairs, allowing the reconstruction of the particle decay position. We present the predictions for the pseudo - rapidity and transverse momentum distributions of the ALPs and photons. Moreover, predictions for the fiducial cross-sections, derived considering the characteristics of the ALICE and LHCb detectors, are presented for different values of the ALP mass and the ALP - photon coupling.

Figures

Figures reproduced from arXiv: 2501.14041 by the authors.

Figure 1
Figure 1. FIG. 1: ALP production and decay into a two - photon system through the [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. FIG. 2: Diagram showing the transverse plane of an ALP decaying into photons outside the luminous region of the PbPb [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. FIG. 3: Distributions of (left) pseudorapidity and (right) transverse momentum of ALP candidates with [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figures from the paper (2 more)
Figure 4
Figure 4. Figure 4: FIG. 4: Distributions of (left) pseudorapidity and (center) transverse momentum of the photons from ALP decays with [PITH_FULL_IMAGE:figures/full_fig_p004_4.png]
Figure 5
Figure 5. Figure 5: FIG. 5: Fiducial cross-sections for ALP production in UPCs for different photon-ALP couplings and ALP masses. [PITH_FULL_IMAGE:figures/full_fig_p005_5.png]

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